As is known, the majority of currently known numeric-control milling machines comprises: a bottom flatbed provided with a horizontal working surface, on which a workpiece is to be positioned; a cutterhead that can move above the working surface in such a way as to be able to carry out milling of the workpiece which is kept stationary on the working surface itself; and, finally, a supporting structure designed to support and displace the cutterhead in space above the working surface in such a way as to enable it to perform all the displacements necessary for carrying out the programmed machining operations.
In the majority of currently known numeric-control milling machines, the supporting structure of the cutterhead consists of: a portal having the shape of a U set upside down, mounted so that it can move on the flatbed of the machine tool in such a way that its horizontal cross member is set astride of the working surface, in a direction perpendicular to the longitudinal axis of the latter; and a head-carrying turret which is mounted mobile on the horizontal cross member of the portal in such a way as to be able to move horizontally in a direction perpendicular to the longitudinal axis of the aforesaid working surface.
The portal can be displaced along the flatbed of the machine tool in a direction parallel to the longitudinal axis of the working surface, thus maintaining the horizontal cross member parallel to itself and perpendicular to the longitudinal axis of the working surface, whilst the cutterhead is mounted mobile on the bottom end of the turret in such a way as to be able to move both in a slewing fashion and in a vertical direction.
Of course, the numeric-control milling machines that adopt the above structural configuration require the use of very rigid portals so as to eliminate any possible errors of spatial positioning of the tool deriving from possible structural deformations of the portal.
Unfortunately, however, the portals that have a structural stiffness adequate for the purpose are intrinsically very heavy and costly, with all the drawbacks that this entails. The use of very heavy portals involves, moreover, the use of electric motors with high specific power, which are particularly costly and present high levels of consumption in terms of electric power, thus contributing to raising the costs of production of the mechanical pieces made with said machines.